This invention relates to facsimile systems comprising a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to a system wherein a document is scanned in a facsimile transmitter to generate electrical information-bearing signals representing dark-light variations in the document being scanned. These information-bearing signals are transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a facsimile of the original document.
Typically, the document at the transmitter circumscribes a rotatably mounted transmitter drum and the copy medium at the receiver similarly circumscribes a rotatably mounted receiver drum. The transmitter drum and the receiver drum are then driven at substantially the same angular frequency so that the area of the document scanned by the optical pickup means equals the overall area of the copy medium traversed by a writing mechanism such as a stylus in a given period of time. However, driving the transmitter drum and the receiver drum at the same angular frequency does not assure that the edge of the document will necessarily correspond with the edge of the copy since the initial position of the optical pickup means with respect to the edge of the document is not necessarily the same as the initial position of the writing mechanism with respect to the edge of the copy medium.
In copending application Ser.No. 333,616 filed Feb. 20, 1973, assigned to the assignee of this invention, the positioning of the edge of the document relative to the optical pickup means and the edge of the copy medium relative to the writing mechanism is not required since the copy medium comprises a continuous loop of material. After a facsimile transmission, the edge of the copy medium may be appropriately located by cutting the closed loop of the copy medium at the proper location. Where a continuous loop of the copy medium is not utilized, it is necessary to synchronize the edge of the document relative to the optical pickup means and the edge of the copy medium relative to the writing mechanism before transmitting the information content of the document at the transmitter to the receiver.
Once transmission of the information content begins, it is necessary to rotate the receiver drum and the transmitter drum at virtually identical frequencies so as to avoid margin skew. Even very slight variations in the frequency of angular drum rotation may result in substantial skewing. For example, variation in frequency at the transmitter of one part in 50,000 combined with a variation in frequency at the receiver of one part in 50,000 will result, assuming the errors are additive, in a combined error of one part in 25,000. This error, assuming it continues throughout the facsimile transmission, will produce a skewing of the copy margin equal to 3/8ths of an inch over the 11 inch length of a document when the drum speed is 180 R.P.M. and the lateral scan rate is 1.8 inches per minute. Such deviations are not at all uncommon where the drum speeds are synchronized with power line frequency and transmitter and receiver are on different power grids. Where the deviations in frequency between the transmitter and receiver do not continue throughout the transmission, e.g., just a matter of seconds, the imperfections in the copy quality are less significant although unsatisfactory imperfections may result where deviations occur for periods of ten seconds or less.
U.S. Pat. No. 3,582,550 - Latanzi discloses a facsimile receiver having a synchronization circuitry for achieving synchronization of the margins on the document and copy medium. The system includes a stable frequency oscillator having an output connected to a frequency generator comprising a number of series-connected flip-flops forming a binary counter. The output of the oscillator is also connected to a clock which determines the elapsed time between transmitter synchronizing pulses and receiver synchronizing pulses. A decoder which is responsive to the period between the synchronizing pulses as determined by the clock selectively adds or deletes pulses in the binary counter so as to alter the frequency of the drive on the receiver drum to secure margin synchronization between the copy medium at the receiver and the document at the transmitter.
In the system of U.S. Pat. No. 3,622,703 - Ricketts, Jr., et al, synchronization of the document and copy medium edges or margins is achieved by driving the receiver drum at a frequency different from the frequency of the transmitter drum. The frequency differential is provided by a voltage controlled oscillator in the receiver. In one system disclosed in the Ricketts, Jr., et al patent, the voltage controlled oscillator is under the control of an error signal generated by a phase detector responsive to the transmitter and receiver synchronizing pulses. When the transmitter and receiver synchronizing pulses achieve a predetermined degree of coincidence, the first-mentioned phase detector becomes inoperative and control of the oscillator is assumed by a phase detector which compares the frequency of the voltage controlled oscillator with that of a fixed frequency crystal oscillator